Methods suitable for the purification of alkanes have long been known in the art. Such methods include hydrogenation to remove unsaturated impurities, adsorbent beds, zone refining, distillation and so forth.
Dumitrescu et al., Romanian Patent RO 101217, discloses the use of hydrogenation for removing unsaturated impurities from n-hexane.
Japanese Patent Application JP 03031304 A (abstract), discloses purification of hexanes by water washing, distillation, treatment in an adsorbent bed, followed by hydrogenation.
Teteruk et al. SO Neftepererabotka i Neftekhimiya (Moscow, Russian Federation) (1988), (7), 18-19 (abstract only), reports a comparison between hydrogenation and adsorption for the purification of hydrocarbons. The report concludes that adsorption is more effective.
Photolithographic methods have been employed for decades to fabricate electronic integrated circuits, and more recently, integrated optical circuit elements. One key enabling technology for fabricating ever-higher density integrated circuits has been the application of shorter and shorter wavelengths of exposure light, the smaller wavelengths permitting resolution of finer lines. Current technology employs ultraviolet (UV) wavelengths, generally below 250 nm, especially at 193 nm, in order to achieve the highest resolution possible in the present state of the art.
Recently it has been found that introduction of a high refractive index liquid in place of air between the photomask and the photosensitive target enables the production of higher resolution images while retaining 193 nm illumination. Switkes et. al., Proceedings of SPIE, Volume 5040, 699(2003) discusses so-called immersion photolithography. Water has been the immersion liquid of choice in photolithography with a 193 nm light source.
Low absorbance of the immersion liquid is of great importance. For a given degree of light transmission to the photosensitive target surface, lower absorbance equates to greater working distance, which is of great practical value. Furthermore, lower absorbance results in less radiative heating of the fluid. Because refractive index is temperature dependent, a change in temperature in the liquid can cause blurring of the image.
Hydrocarbons, especially alkanes, are known to exhibit refractive indices higher than that of water. For example, replacement of water as an immersion liquid by bicyclohexyl, with a refractive index of 1.64, would reduce the effective wavelength of 193 nm light to 118 nm. However, to be of practical use, immersion liquids must also be quite transparent. The absorbance requirements suitable for practical use appear to be ever tightening. For example, Zhang et al., U.S. Published Patent Application 2005/0173682, describe immersion fluids characterized by absorbance of 5 cm−1 whereas today, practical absorbance is thought to lie at ≦0.10 cm−1.
Miyamatsu et al., WO2005/114711 (examples and claims only), disclose a process for preparing highly transparent alkanes by a combination of treatment with sulfuric acid and distillation.
French et al., WO2005/119371 discloses methods for purifying alkanes to achieve high transparency including hydrogenation and adsorbent treatment.